1. Field of the Invention
The present invention relates to a satellite telecommunications and remote sensing system, based on the use of a number of satellites placed in elliptical sun-synchronous orbits, inclined with respect to the equatorial plane, and having an orbital period of approximately three hours.
The system according to the invention may be used advantageously for telecommunication via satellite when it is necessary to settle simplex or duplex radio communication within a country or geographical areas which are homogeneous as regards language, culture, ethnicity or socio-economic interests, or between countries situated in non-contiguous geographical areas. The telecommunications services, for which the invention provides significant economical advantages, include, among other things: personal communications; communication through mobile ground, sea and air means; remote radio-location of the position of the mobile means, and radio navigation; personal selective remote indication; simplex or duplex data transmission between small fixed or portable terminals; direct radio broadcasting of radio or TV programs; teleconferences; emergency communications; and data dissemination and collection.
The invention is also applicable to regional remote sensing, in particular in the field of meteorology and Earth observation using optical sensors.
On the basis of the invention, the sun-synchronization features of the satellite orbit and the position of the orbital plane (essentially according to the normal to the solar rays), in comparison with other satellite systems used for the same applications, make it possible to simplify the satellite configuration, especially as regards: power generation on the part of the solar generation; storage of electrical power through rechargeable batteries; and thermal conditioning of the satellite.
The inclination of the orbital plane with respect to the equator is equivalent to 116.4.degree. in order to annul the rotation of the line of apsides. The eccentricity of the orbit and the orbital period, which results from the choice of a sun-synchronous orbit with an inclination of 116.4.degree., together with the value of the inclination and an orientation of the position of the orbital plane such as to result almost transversal to the direction of the solar rays, provide the following positive effects:
the satellite is visible for sufficiently long periods of time at the apogees, and on average around 12:00 hours or 24:00 hours, local time; and PA1 the satellite is visible from ground stations located at latitudes greater than approximately 20.degree. North or smaller than approximately 20.degree. South, with sufficiently large angles of elevation with respect to the horizon and such as to allow radio links which are less dependent on atmospheric precipitations and on blocking phenomena caused by natural or artificial obstacles. PA1 low circular orbits (LEO=Low Earth Orbits), with orbital altitudes usually in the range of 500 to 1,500 Km (for remote sensing purposes, the orbital altitude is usually below 1,000 Km); PA1 slightly elliptical orbits with a very short period, i.e. usually less than 2 hours, with an eccentricity ranging from 0.05 to 0.15; PA1 medium-period circular orbits, with altitudes above 10,000 Km; and PA1 elliptical orbits inclined by 63.4.degree. with respect to the equatorial plane, with 24-hour (Tundra orbit), 12-hour (Molniya orbit) or 8-hour periods. PA1 medium-high orbital altitudes, at least for most of the orbital period, so as to considerably increase the duration of the satellite visibility windows from the ground stations, and the duration of observability of the same geographical area from the satellite; PA1 an orbital period intermediate between that of the-low orbits and that of the medium-period orbits, so as to limit the cost of injecting the satellite into orbit; PA1 reduced crossing through the Van Allen belts, so as to reduce damage to the electronic circuits; and PA1 the possibility of building simpler satellites, by exploiting to the utmost the solar lighting conditions of the satellite during the whole year. PA1 1. definition of a family of orbits particularly convenient for the implementation of telecommunication and remote sensing systems; PA1 2. definition of multisatellite constellations based on the particular features of this family of orbits, and of one orbit in particular; and PA1 3. definition of the general characteristics of a satellite capable of exploiting, in an economically convenient manner, the peculiar characteristics of these orbits in the implementation of the satellite systems. PA1 in the range of latitudes going from approximately 20.degree. to 70.degree. North, which contains industrialized countries and therefore the most interesting countries as regards the current demand for services and their profitability; and PA1 in the range of latitudes going from 20.degree. to 70.degree. South, and in the tropical band included between approximately 20.degree. North and 20.degree. South, where developing countries or recently industrialized countries are situated, which are interesting as regards the expected rate of growth in the demand for telecommunication and remote sensing services.
2. Description of the Prior Art
The currently available satellite telecommunication and remote sensing systems have so far exploited stationary orbits as well as different types of non-stationary orbits. Among the latter, the most worthy of mention are:
The stationary orbits used in the past, and still proposed today in creating innovative telecommunication systems, were not based on the use of sun-synchronous orbits or on the use of eccentricity values such as to obtain orbital periods lasting approximately 3 hours.
In particular:
Low circular or slightly elliptical short-period orbits have been used for constellations with a large number of satellites (from 12 to 80) for the coverage of the entire globe. The services rendered include telephony, personal communications and communications through mobile devices. The main inconveniences regarding these constellations reside in the large number of satellites, inversely proportional to the orbital altitude; and the reduced visibility--approximately 10 minutes--of each satellite on the part of the ground user. In remote sensing applications, the short fly times over the area to be surveyed made it practically impossible to carry out observation missions which, on the contrary, require long observation times of the same geographical area from the satellite. The only practical possibility, for these missions, was represented by the use of stationary orbit satellites. However, in this case, the great distance of the satellite from Earth involved the creation of large-sized optical systems in order to obtain satisfactory performance in terms of geometric and radiometric resolution. Circular medium-period orbits require a smaller number of satellites for the continuous coverage of the globe when compared to low orbits; however, this advantage is offset by the fact that these orbits have a greater energy requirement than the lower orbits. For this reason, injecting satellites into their target orbits is more expensive than injecting them into low or slightly elliptical orbits. Coverage requirements also require that these orbits have orbital plane inclinations with respect to the equator ranging from 40.degree. to 60.degree. and are therefore characterized by a rotation of the nodal line such that the position of the orbital plane with respect to the direction of the solar rays changes continuously during the year. This implies a continuous variation of the electrical power coming from the on-board solar plant, and this fact is made even more critical by the occurrence of blackout periods when the satellite enters into the shadow cone of the Earth. There is also a continuous change in the environmental thermal conditions experienced by the satellite during its operational life. All of these factors imply a higher weight and a greater complexity for the satellite, as well as an increase in its design and production costs. Long-period elliptical incline orbits are used in telecommunications satellite systems for the coverage of single continents or for well-defined geographical areas. These orbits have a 63.4.degree. inclination of the orbital plane, since with this value of inclination the rotation of the line of apsides is annulled, and therefore the orbit apogee and perigee always correspond to the same terrestrial latitudes. The orbital period is chosen equal to the sidereal day (Tundra orbit) or to a submultiple of the sidereal day (Molniya orbit and 8-hour orbit), so that the satellite motion is synchronous or subsynchronous with .the rotation of the Earth. While this ensures the periodical presentation of the satellite over the same geographical areas, it also involves a rotation of the nodal line which makes the position of the orbital plane vary in time with respect to the direction of the solar rays, thus causing the same inconveniences as those mentioned for medium-period circular orbits.